Ultrahigh molecular weight polyethylene (hereinafter referred to as "UHMW polyethylene") is commonly used to make prosthetic joints such as artificial hip joints. In recent years, it has become increasingly apparent that tissue necrosis and interface osteolysis, in response to UHMW polyethylene wear debris, are primary contributors to the long-term loosening failure of prosthetic joints. For example, the process of wear of acetabular cups of UHMW polyethylene in artificial hip joints introduces many microscopic wear particles into the surrounding tissues. The reaction of the body to these particles includes inflammation and deterioration of the tissues, particularly the bone to which the prosthesis is anchored. Eventually, the prosthesis becomes painfully loose and must be revised. It is generally accepted by orthopaedic surgeons and biomaterials scientists that the reaction of tissue to wear debris is the chief cause of long-term failure of such prostheses.
Laboratory experiments and examination of worn polyethylene components, as used in acetabular cups of total hip prostheses, after removal from patients, have shown that polyethylene wear in vivo primarily involves three fundamental mechanisms: adhesive, abrasive, and fatigue wear {Brown, K. J., et al., Plastics in Medicine & Surgery Plastics & Rubber Institute, London, 2.1-2.5 (1975); Nusbaum, H. J. & Rose, R. M., J. Biomed. Materials Res., 13:557-576 (1979); Rostoker, W., et al., J. Biomed. Materials Res., 12:317-335 (1978); Swanson, S. A. V. & Freeman, M. A. R., Chapter 3, "Friction, lubrication and wear.", The Scientific Basis of Joint Replacement, Pittman Medical Publishing Co., Ltd. (1977).}
Adhesive wear occurs when there is local bonding between asperities on the polymer and the opposing (metal or ceramic) counterface. If the ratio of the strength of the adhesive bond to the cohesive strength of the polymer is great enough, the polymer may be pulled into a fibril, finally breaking loose to form a wear particle. Small wear particles (measuring microns or less) are typically produced.
Abrasive wear occurs when asperities on the surface of the femoral ball, or entrapped third-body particles, penetrate into the softer polyethylene and cut or plow along the surface during sliding. Debris may be immediately formed by a cutting process, or material may be pushed to the side of the track by plastic deformation, but remain an integral part of the surface.
Fatigue wear is dependent on cyclic stresses applied to the polymer. As used herein, fatigue wear is an independent wear mechanism involving crack formation and propagation within the polymer. Cracks may form at the surface and coalesce, releasing wear particles as large as several millimeters and leaving behind a corresponding pit on the surface, or cracks may form a distance below the surface and travel parallel to it, eventually causing sloughing off of large parts of the surface.
There are gaps in the prior art regarding the contributions of the above three basic mechanisms to the wear of polyethylene cups in vivo. While numerous laboratory studies and analyses of retrieved implants have provided valuable details on wear in vivo, there is ongoing disagreement regarding which wear mechanisms predominate and what are the controlling factors for wear.
However, it is clear that improving the wear resistance of the UHMW polyethylene socket and, thereby, reducing the amount of wear debris generated each year, would extend the useful life of artificial joints and permit them to be used successfully in younger patients. Consequently, numerous modifications in physical properties of UHMW polyethylene have been proposed to improve its wear resistance.
UHMW polyethylene components are known to undergo a spontaneous, post-fabrication increase in crystallinity and changes in other physical properties. {Grood, E. S., et al., J. Biomedical Materials Res., 16:399-405 (1976); Kurth, J., et al., Trans. Third World Biomaterials Congress, 589 (1988); Rimnac, C. M., et al., J. Bone & Joint Surgery, 76-A(7):1052-1056 (1994)}. These occur even in stored (non-implanted) cups after sterilization with gamma radiation which initiates an ongoing process of chain scission, crosslinking, and oxidation or peroxidation involving free radical formation. {Eyerer, P. & Ke, Y. C., J. Biomed. Materials Res. 18:1137-1151 (1984); Nusbaum, H. J. & Rose, R. M., J. Biomed. Materials Res., 13:557-576 (1979); Roe, R. J., et al., J. Biomed. Materials Res., 15:209-230 (1981); Shen, C. & Dumbleton, J. H., Wear, 30:349-364 (1974)}. These degradative changes may be accelerated by oxidative attack from the joint fluid and cyclic stresses applied during use. {Eyerer, P. & Ke, Y. C., J. Biomed. Materials Res., supra; Grood, E. S., et al., J. Biomed. Materials Res., supra; Rimnac, C. M., et al., ASTM Symposium on Biomaterials' Mechanical Properties, Pittsburgh, May 5-6 (1992)}.
On the other hand, it has been reported that the best total hip prosthesis for withstanding wear is one with an alumina head and an irradiated UHMW polyethylene socket, as compared to a un-irradiated socket. The irradiated socket had been irradiated with 10.sup.8 rad of .gamma.-radiation, or about 40 times the usual sterilization dose. {Oonishi, H., et al., Radiat. Phys. Chem., 39(6):495-504 (1992)). The usual average sterilization dose ranges from 2.5 to 4.0 Mrad. Other investigators did not find any significant reduction in the wear rates of UHMW polyethylene acetabular cups which had been irradiated, in the solid phase, in special atmospheres to reduce oxidation and encourage crosslinking. {Ferris, B. D., J. Exp. Path., 71:367-373 (1990); Kurth, M., et al., Trans. Third World Biomaterials Congress, 589 (1988); Roe, R. J., et al., J. Biomed. Materials Res., 15:209-230 (1981); Rose, et al., J. Bone & Joint Surgery, 62A (4):537-549 (1980); Streicher, R. M., Plastics & Rubber Processing & Applications, 10:221-229 (1988)}.
Meanwhile, DePuy.DuPont Orthopaedics has fabricated acetabular cups from conventionally extruded bar stock that has previously been subjected to heating and hydrostatic pressure that reduces fusion defects and increases the crystallinity, density, stiffness, hardness, yield strength, and resistance to creep, oxidation and fatigue. (U.S. Pat. No. 5,037,928, to Li, et al., Aug. 6, 1991; Huang, D. D. & Li, S., Trans. 38th Ann. Mtg., Orthop. Res. Soc., 17:403 (1992); Li, S. & Howard, E. G., Trans. 16th Ann. Society for Biomaterials Meeting, Charleston, S.C., 190 (1990).} Silane cross-linked UHMW polyethylene (XLP) has also been used to make acetabular cups for total hip replacements in goats. In this case, the number of in vivo debris particles appeared to be greater for XLP than conventional UHMW polyethylene cup implants {Ferris, B. D., J. Exp. Path., 71:367-373 (1990)}.
Other modifications of UHMW polyethylene have included: (a) reinforcement with carbon fibers {"Poly Two Carbon-Polyethylene Composite-A Carbon Fiber Reinforced Molded Ultra-High Molecular Weight Polyethylene", Technical Report, Zimmer (a Bristol-Myers Squibb Company), Warsaw (1977)}; and (b) post processing treatments such as solid phase compression molding {Eyerer, P., Polyethylene, Concise Encyclopedia of Medical & Dental Implant Materials, Pergamon Press, Oxford, 271-280 (1990); Li, S., et al., Trans. 16th Annual Society for Biomaterials Meeting, Charleston, S.C., 190 (1990); Seedhom, B. B., et al., Wear, 24:35-51 (1973); Zachariades, A. E., Trans. Fourth World Biomaterials Congress, 623 (1992)}. However, to date, none of these modifications has been demonstrated to provide a significant reduction in the wear rates of acetabular cups. Indeed, carbon fiber reinforced polyethylene and a heat-pressed polyethylene have shown relatively poor wear resistance when used as the tibial components of total knee prosthesis. {Bartel, D. L., et al., J. Bone & Joint Surgery, 68-A(7):1041-1051 (1986); Conelly, G. M., et al., J. Orthop. Res., 2:119-125 (1984); Wright, T. M., et al., J. Biomed. Materials Res., 15: 719-730 (1981); Bloebaum, R. D., et al., Clin. Orthop., 269:120-127 (1991); Goodman, S. & Lidgren, L., Acta Orthop. Scand., 63(3) 358-364 (1992); Landy, M. M. & Walker, P. S., J. Arthroplasty, Supplement, 3:S73-S85 (1988); Rimnac, C. M., et al., Trans. Orthopaedic Research Society, 17:330 (1992); Rimnac, C. M. et al., "Chemical and mechanical degradation of UHMW polyethylene: Preliminary report of an in vitro investigation," ASTM Symposium on Biomaterials' Mechanical Properties, Pittsburgh, May 5-6 (1992)}.